Manufacturing factories, assembly plants, buildings with temperature control, semiconductor fabrication plants, facilities with energy management systems, and process plants have many systems and operation processes that add to the efficient operation of one or more associated control, maintenance and factory automation systems. Many of the operational processes can be monitored to ensure proper equipment and process functioning and to provide feedback for process control, maintenance (including predictive maintenance), supervisory control and data acquisition (SCADA), and test and automation systems to identify process problems or potential equipment failures. Monitored processes can include the measurement of temperature, flow, pressure, level, current, power, motion, vibration, fluid properties and equipment failure and related data, among others, that are monitored by sensors.
Electronic sensors require additional signal processing of the sensed variable including the transducing of the variable or other characteristics that are transduced into an electrical voltage, current, resonant frequency or digital word that is indicative of the characteristic and which can be transmitted to systems. The signal processing is usually accomplished by a transmitter connected to the sensor or through electronics directly connected to the sensor by a wire or cable or connected wirelessly.
As one example, temperature is an often measured physical variable or characteristic of manufacturing processes. The most common temperature measurement systems utilize thermocouples (TCs), resistance temperature devices (RTDs), thermopiles and/or thermistors to sense the physical temperature variable. Other temperature sensors can include, by way of example, semiconductor based RTDs, diodes, infrared sensors, and resonant quartz sensors.
A typical sensor is wired directly to a single channel of an input/output (I/O) device. This direct wiring method is commonly referred to as point-to-point wiring. In this case, the temperature sensor provides an electrical output where an electrical parameter such as a resistance or a voltage changes with changes in a repeatable manner with temperature. The I/O device then converts the electrical parameter into a standard output compatible with a controller or the input requirements of a monitoring or controlling device. The output of the I/O device can be analog, such as a voltage or a current, or can be digital signal or code such as one that conforms to a digital bus standard such as Ethernet TCP/IP, EtherNet/IP, FDDI, ControlNet, Modbus™ (a trademark of AEG Schneider Corporation), Profibus, ProfiNet, IEEE 802.XX wireless and various fieldbuses or proprietary network protocols. The I/O device used for temperature systems typically converts only one parameter from a temperature sensor such as the parameter that provides the physical variable, temperature. In addition to converting the temperature sensor electrical parameter into a standard output that can be interfaced to a controller or a monitoring or controlling system, the I/O device can also provide additional signal processing. Such signal processing can provide for linearization of the sensor, increasing the output level of the sensor (gain), removing stray electrical noise, and/or providing isolation from stray electrical currents. Traditionally temperature I/O devices are provided by manufacturers such as Action Instruments and Rochester Instruments.
The functions of simple signal processing of temperature sensor electrical parameters are well known and are performed by many manufacturers of temperature I/O device products. One common temperature sensing system uses a transmitter located adjacent to a thermal process. Transmitters are used in approximately 50% of all thermal systems and provide signal processing of the electrical parameter, electrical isolation of the sensor from stray electrical and mechanical noise inputs, linearization and scaling of the electrical output, a local means for calibrating the sensor and a standardized output. Traditional standardized outputs for transmitter-based systems include a two-wire 4-20 mA output, a 10-50 mA output, and a 1-5V output. Traditional temperature transmitters are manufactured by companies such as Rosemount, ABB Hartmann & Braun, and Honeywell. Transmitter based systems are also wired to the I/O device, but the I/O device accommodates only standardized voltage or current inputs and does not provide additional signal processing of the temperature variable. Commercial standard I/O devices are provided by programmable logical controller suppliers such as Siemens, Allen Bradley and Omron. Digital control systems suppliers include Emerson Process, Honeywell, Siemens, Invensys, Yokogawa, and ABB. Third parities often provide input/output interfaces for digital control systems (DCS) (such as a control system used in a process control plant such as chemical, refining, electric power where the materials flow continuously in pipes) or programmable logic controllers (PLC)'s such as Opto22, Moore Industries, Action Instruments, and Phoenix (PLCs are often used in factories for the control of discrete events like the manufacture of automobiles or widgets or process plants where products are made in batches, like pharmaceuticals). The I/O device can perform a multiplexing function and can convert the temperature signal to a standardized output that can be used by a controller, control systems, or monitoring system. Further, the output from the I/O device is most commonly a digital signal on a fieldbus.
Another form of a sensor-transmitter based monitoring system includes a microprocessor located at or near the sensor. These “smart transmitters” were first introduced in the 1980s and have the ability to output or communicate a digital message over a bus. Smart transmitters provide for improved signal processing and linearization using the microprocessor with embedded or related software programs. In addition, a digital output is communicated over a bus and enables the smart transmitters to be wired in a “multi-dropped” fashion that reduces the wiring and the number of I/O channels for a particular operational application. Smart transmitters can communicate digitally to an I/O using HART®, Foundation Fieldbus, Profibus PA, or proprietary protocols such as the Honeywell DE, Yokogawa Brain, or Foxboro I/A.
The I/O device used with a smart transmitter converts the digital input from a transmitter to a digital output (usually at a higher baud rate and/or different protocol) that is compatible with a controller connected to the output of the I/O device. The digital I/O device does not normally provide for signal conditioning of the transmitter variable, but functions as a data concentrator and protocol converter. The integral power supply in the I/O can often provide electrical power over the wire to the transmitters.
With the introduction of digital smart sensors in the 1980's, the transmitter and controller manufacturers introduced a plethora of digital communication protocols often referred to as fieldbuses. Fieldbus protocols can be used with several types of physical media including 2 and 4 wire, optical media, wireless, etc. Generally, the lowest speed fieldbus provides a high speed two wire communication protocol that can be used to digitally integrate sensors, actuation devices, controls, monitoring systems and equipment with an operations or management system. The fieldbus is characterized by low power consumption and 32 bit messaging capability built on a standard open protocol. Fieldbus transmitters and systems provide higher speed (baud rate) buses and have the ability to handle large amounts of data. A typical data rate for a fieldbus is 31.25 Kbps. Fieldbuses are also limited to the delivery of relatively low levels of electrical power to assure that the fieldbus-based transmitters satisfy electrical power requirements of intrinsically safe or increased safety electrical industry standards required for safe operation in process environments where explosive gases may be present.
A fieldbus-based transmitter can be connected to a control system and an enterprise asset management (EAM) system that uses diagnostics information from the transmitter for process, system, transmitter or equipment diagnostics. The fieldbus transmitter has the ability to provide large amounts of data at data rates of up to 32 kb/s while limiting its power consumption to low levels as required for intrinsically safe systems. Typically, manufacturers provide diagnostics information from their transmitter products. The diagnostics information is produced by the local transmitter by processing electrical parameters originating from the sensor to provide diagnostics related to the sensor, the process, the sensor wiring, the transmitter electronics, the wiring, and the digital fieldbus. This diagnostics information usually has standard diagnostic parameters as well as diagnostic parameters that are different for each manufacturer, but is readily communicated digitally to and from the fieldbus transmitter over the standard fieldbus to a fieldbus compatible I/O device. The cost of the fieldbus transmitters are often more than twice the price of traditional transmitters and are ten to twenty times the price of a sensor.
One fieldbus manufacturer, Rosemount, Inc., developed a protocol that is called HART® (Highway Addressable Remote Transducer) that is a hybrid protocol. The HART® protocol provides for a frequency shift keying (FSK) digital signal superimposed over the standard 4-20 mA analog wiring formerly used as the transmitter standard. Rosemount, Inc. donated the HART® protocol to the public domain and HART® is now available to all manufacturers through The HART® Communication Foundation (HCF). HART® is now the most frequently used transmitter protocol. A smart HART®-type transmitter can replace traditional 4-20 mA transmitters without changes in the wiring or I/O device if the user wants only a temperature parameter. HART® can also provide digital information with an associated change in the I/O device. With the advent of the smart digitally communicating transmitters, additional information can also be communicated from the transmitter such as field device tag number, manufacturer's ID, scaling factor, simple electronics diagnostics.
Other digital protocols have also emerged such as Modbus™ and, most recently higher speed protocols with the ability to provide distributed computing have been introduced. Some fieldbus protocols such as the Foundation fieldbus, ProfiNet and Profibus PA provide for standardized messaging and parameters for diagnostics. Other systems provide a diagnostic capability such as a self-validating sensor (SEVA) that includes diagnostics alerts.
Fieldbus transmitters are often more than twice as expensive as traditional transmitters, due in part to the more sophisticated electronics required to process more parameters and to the complexity of the communications signal processing for a fieldbus. Many operational or process systems do not require the full capabilities of the fieldbus and do not justify the high implementation cost, especially when monitoring simple, less-costly sensors, actuation devices, and discrete devices such as contact closures, switches, and/or digital on-off devices. As such, there is a need for a simple, less costly system and method that provides many of the higher level diagnostic functions available with the fieldbus technology.
One such alternative for a lower cost operational communication system is the “bitbus.” A bitbus typically provides 8 bits of messaging information. A bitbus can carry relatively high speed messaging at a low cost. One such bitbus is the DeviceNet™ (a trademark of Open DeviceNet™ Vendors Association (ODVA)) that is an extension of an automotive digital bus, the Car Automation Network (CAN). A second bitbus, AS-Interface (AS-i), was developed in Europe for simple digital or logical inputs for factory automation systems and is, similarly, very low cost.
Generally, current bitbus-based devices do not include extensive diagnostic capabilities, do not provide signal conditioning that is often required in a harsh operating environment, and do not interface well with higher level operational systems. Due to the limited bandwidth of the bitbus, current bitbus systems do not include digital diagnostics information processing of diagnostic parameters that are contained in the digital bitbus message from a sensor or actuating device.